FIELD OF THE INVENTION
The present invention relates to liquid chromatography systems generally, and more particularly to systems for conducting chromatographic analysis at high temperatures. This invention also relates to methods for performing chromatographic analysis on liquid samples at elevated or high temperatures.
A number of liquid chromatography systems are in use today, which systems utilize a variety of configurations specifically tailored to particular chromatographic applications. In many of such applications, elevated temperatures have been determined to be helpful in the elution of liquid samples in mobile phases. As a general matter, increased temperature of the liquid mobile phase correspondingly lowers mobile phase viscosity, which allows an increased mobile phase flow rate through the liquid chromatography system while maintaining desired chromatographic analysis attributes. As a result, a number of liquid chromatography systems in use today utilize heating means for elevating the mobile phase temperature as the mobile phase is directed through the system.
Liquid chromatography heating instrumentation and design has typically been confined to the temperature range of ambient to 60xc2x0 C., and flow rates from zero to three milliliters per minute, as dictated by the particular materials making up the chromatographic systems. A specific limitation to existing chromatographic systems for processing mobile phase streams at elevated temperatures is the packing material utilized in the liquid chromatography columns. Such packing material is typically a silica or less typically a polymer-based material. Silica-based materials are chemically and thermally unstable at temperatures above 100xc2x0 C., while polymeric materials tend to swell or change shape causing problems in use. Therefore, more temperature-resistant materials must be utilized in order to allow chromatographic analysis of liquid mobile phases above 60xc2x0 C.
An example of such a thermally-stable material is zirconia which provides relatively stable analytical separations at temperatures even in excess of 200xc2x0 C. In fact, recent tests have demonstrated that packing materials utilizing zirconia as the substrate material are chemically and thermally stable at temperatures approaching the critical point of water (375xc2x0 C.).
The significantly raised temperature limits of the mobile phase in liquid chromatography systems made possible by such packing materials provide a number of advantages over typical, relatively low-temperature ( less than 60xc2x0 C.) chromatography systems. For example, high-temperature mobile phase liquids reflect a correspondingly lower viscosity, such that flow rate through the chromatographic column may be increased while maintaining a substantially laminar flow regime. In addition, advantageous solvent properties may be realized at such elevated or high temperatures. Water, for example, increasingly resembles an organic solvent as temperature increases toward the critical temperature of water. In fact, recent tests and calculations indicate that at 250xc2x0 C., water exhibits solvent properties approaching those of the pure organic solvents most commonly used in liquid chromatography applications, such as methanol and acetonitrile. Thus, in reversed phase applications, the transfer of the solute from pure water to the stationary phase at high temperature (200xc2x0 C.) resembles that of the transfer of a solute from a pure organic eluent to the stationary phase at 25xc2x0 C. The use of only water as a mobile phase is environmentally and economically highly desirable. Further, the decrease in viscosity of water at temperatures above 100xc2x0 C. may be exploited by substantially increasing mobile phase flow rates, as compared to standard temperature chromatographic systems, thereby substantially decreasing analysis time. Such flow rate increases are made possible by the lower viscosity which correspondingly decreases the back pressure of the mobile phase within the chromatographic column. The decreased back pressure allows increased throughput flow rate without exceeding the mechanical pressure limits of the liquid chromatograph pumping system. A further advantage of high-temperature chromatography is providing the analyst additional means to optimize sample separation and increase resolution of various analytes.
Chromatographic heating systems in use today, however, are generally operated below 60xc2x0 C., and as such have a number of disadvantages which compromise the overall efficacy of such high-temperature liquid chromatography. Some existing systems utilize conductive or convective heating to the chromatographic column to impart heat energy to the mobile phase for elevated temperature analysis of samples dissolved therein. Such techniques fail to properly xe2x80x9cpre-heatxe2x80x9d the mobile phase prior to admission into the chromatographic column, whereby mobile phase temperature profiles are created radially and axially within the chromatographic column. Mobile phase temperature profiles are, in general, undesired in liquid chromatography applications, as such temperature profiles typically result in peak broadening.
Some chromatographic heating systems utilize a radiant or convective oven in which some of the chromatographic instruments are placed for elevating the temperature of the mobile phase being transported to the column. Such ovens are typically relatively large in volume to encompass at least a portion of the chromatographic system in a heated environment, and have varied success in elevating respective temperatures uniformly. For example, the desired temperature may not be reached in all locations within the oven, such that the temperature within respective chromatographic instruments may vary depending upon their positions within the oven. In addition, the temperature within the chromatographic column can vary both radially and axially, due to differences in temperature of the incoming mobile phase as compared to that of the oven. A common problem experienced with oven heating systems is the column temperature varying from the desired temperature set point, due either to temperature gradients within the oven or slow thermal equilibration of the column under actual operating conditions.
One method utilized to minimize such temperature gradient conditions is the use of pre-heater devices for elevating the temperature of the mobile phase before directing the mobile phase into the chromatographic column. Such pre-heaters may be in a variety of forms, though most typically a means for imparting a pre-determined amount of heat energy through conductive or convective means is utilized. Because such pre-heaters are typically programmed to provide a pre-defined amount of heat energy to the mobile phase, adjustment for varying environmental conditions and incoming mobile phase temperatures for elevating the mobile phase to a desired temperature set point is not well accomplished by existing systems. Furthermore, such pre-heaters are typically not optimized to deliver the heated mobile phase to the chromatographic column at a temperature consistent with gradient-free adiabatic conditions within the column.
It is therefore a principle object of the present invention to provide a system for performing liquid chromatographic analysis at elevated temperatures, wherein temperature gradients in the chromatographic column are minimized.
It is another object of the present invention to provide a chromatographic system for analyzing samples in mobile phases heated above 100xc2x0 C.
It is a further object of the present invention to provide a high-temperature chromatographic system which utilizes heated mobile phases in a substantially adiabatic environment through a chromatographic column.
It is a yet further object of the present invention to provide a high-temperature liquid chromatography system in which external energy required to sufficiently heat the mobile phase is minimized.
It is a still further object of the present invention to provide a high-temperature liquid chromatography system having counter-flow heat exchange means for utilizing heat energy stored in mobile phase exiting the chromatographic column.
It is another object of the present invention to provide a load-responsive system which is capable of dynamically adjusting energy input at various locations in the system to achieve desired set point temperatures in various mobile phase flow regimes.
Another object of the present invention is to provide a high-temperature liquid chromatography apparatus having one or more temperature-control means for maintaining respective chromatographic instruments at desired temperature set points.
It is a still further object of the present invention to provide a liquid chromatography system having temperature stabilizing means operably coupled to a chromatographic column for maintaining a substantially adiabatic environment between the column and the heated mobile phase passing therethrough.
It is a further object of the present invention to provide a liquid chromatographic system having one or more temperature-sensing means for regulating various temperatures in the system, including the elevated-temperature mobile phase.
It is a still further object of the present invention to provide a high-temperature liquid chromatography system utilizing, in combination, a counter-current heat exchanger placing inlet and outlet mobile phase in thermal contact with one another, a pre-heater apparatus for elevating the inlet mobile phase to a desired set point temperature, and a temperature stabilizing device operably coupled to a respective chromatographic column for maintaining a substantially adiabatic environment between the column and the mobile phase passing therethrough, such that temperature gradients within the column are minimized.
By means of the present invention, an improved high-temperature liquid chromatography apparatus is provided for analyzing samples utilizing or dissolved within liquid mobile phases subcritical at temperatures in excess of 100xc2x0 C. The system of the present invention is preferably configured to minimize temperature gradients within the chromatographic column, such that a substantially adiabatic environment is obtained between the column and the elevated-temperature mobile phase passing therethrough. By performing such liquid chromatography at temperatures exceeding 100xc2x0 C., it has been found that it is possible to decrease the proportion of organic solvents in aqueous mobile phases, or, depending on the sample, utilize water as a sole solvent in the mobile phase, which correspondingly provides economic and environmental benefits over the existing use of organic solvents in liquid chromatography applications.
One embodiment of the system for conducting high-temperature liquid chromatographic analysis includes a mobile-phase transport tube configured to operably convey mobile phase from a mobile phase source to respective chromatographic instruments in the system, a pre-heater apparatus operably coupled to the mobile phase transport tube for heating the mobile phase to a desired temperature, and a chromatographic column operably coupled to, and disposed downstream from, the pre-heater apparatus, wherein the chromatographic column includes temperature stabilizing means associated therewith for maintaining the column in a substantially adiabatic environment with the mobile phase passing therethrough. The pre-heater apparatus may include a conductive heating element disposed adjacent to the mobile phase transport tube, and one or more temperature sensing means adapted to determine the temperature of respective components of the pre-heater apparatus, as well as the mobile phase therein. The temperature sensing means are preferably operably coupled to temperature control means, or power input control means, which are adapted to regulate the power input to the heating element so as to maintain the respective elements at predetermined set point temperatures. The chromatographic column preferably includes a column heating apparatus for maintaining the column at a desired set point temperature, wherein the column set point temperature is substantially equal to the mobile phase temperature entering the column.
Another embodiment of the high-temperature chromatography system includes a mobile phase inlet conduit configured to operably convey mobile phase from a mobile phase source to the system, a pre-heater apparatus operably coupled to the mobile phase inlet conduit for heating the mobile phase to a desired temperature, a chromatographic column operably coupled to the pre-heater apparatus, wherein the chromatographic column includes insulation means for maintaining the column at a temperature consistent with the mobile phase passing therethrough, and a mobile phase outlet conduit configured to operably convey the mobile phase from the column, a portion of the outlet conduit being disposed in propinquant relationship with a portion of the inlet conduit in a heat exchange zone, such that a counter-flow heat exchanger is created between respective portions of the inlet and outlet conduits in the heat exchange zone, whereby heat contained in the outlet mobile phase is conductively transferred to the inlet mobile phase.
The present invention also contemplates a method for analyzing liquid samples in a high-temperature chromatographic environment. The method includes providing a mobile phase inlet conduit configured to operably convey mobile phase from a mobile phase source to respective chromatographic instruments, providing a pre-heater apparatus operably coupled to the mobile phase inlet conduit for heating the mobile phase to a desired temperature, providing a chromatographic column operably coupled to the pre-heater apparatus, whereby the column includes insulation means for maintaining the column at temperatures consistent with the mobile phase passing therethrough, and providing a mobile phase outlet conduit configured to operably convey the mobile phase from the column, wherein a portion of the outlet conduit is disposed in a thermally conductive relationship with a portion of the inlet conduit in a heat exchange zone, such that counter-flow heat exchanger is created between respective portions of the inlet and outlet conduits in the heat exchange zone. The method includes utilizing a mobile phase pump to inject mobile phase into the mobile phase inlet conduit and through respective chromatographic instruments, and allowing heat to dissipate into the inlet mobile phase from the outlet mobile phase in the heat exchange zone through conductive heat transfer. The mobile phase is then further heated in the pre-heater apparatus to a first pre-determined set point, and thereafter the sample dissolved within the mobile phase is chromatographically separated in the column at an elevated temperature of at least 100xc2x0 C., which elevated temperature is maintained in a substantially adiabatic state throughout an entire length of the column. The mobile phase is then cooled in the heat exchange zone through conductive heat transfer to the inlet mobile phase. The cooled and chromatographically separated sample is then analyzed in an appropriate chromatographic detector downstream from the heat exchange zone.